1. Field of the Invention
This invention pertains to the removal and recovery of sulfur from various industrial gas streams, in particular those gas streams containing elemental sulfur. Special utility is seen in the processing of flue gases in which sulfur dioxide has been reduced to elemental sulfur.
2. Background of the Invention and Description of Related Art Including Information Disclosed Under 37 CFR 1.97-1.99
Acid rain is thought to be caused by reaction between water and sulfur oxides and/or nitrogen oxides to produce the corresponding acids in the atmosphere. It is believed that sulfur oxides enter the environment in large part on combustion of sulfur-containing coal, and to a lesser extent oil and natural gas, in electric power generating plants and other industrial plants. The problem is serious and substantial attention has been devoted to its solution. Semrau reviewed the topic of industrial process sources of sulfur oxides in Advances in Chemistry Series 139 1-22 (1974).
A well known and probably preferred integrated catalytic process for removing sulfur oxides from the flue gas streams and recovering elementary sulfur comprises adjusting the redox conditions in or near the furnace so as to reduce the sulfur oxides to hydrogen sulfide. The hydrogen sulfide is thereafter extracted from the flue gas and sent to an elemental sulfur recovery step such as the Claus process wherein the hydrogen sulfide is oxidized to water and elemental sulfur. Brocoff in U.S. Pat. No. 4,012,488 teaches the use of hydrocarbons such as methane to reduce sulfur dioxide to hydrogen sulfide. Other processes oxidize hydrogen sulfide to elemental sulfur in solution, as disclosed, for example, by Nicklin et al in U.S. Pat. Nos. 2,997,439, 3,035,889 and 3,097,926. Gorlich in U.S. Pat. Nos. 4,874,589, and 4,880,606 also teaches the oxidation in solution of hydrogen sulfide to elemental sulfur using oxygen in the presence of 1,4-naphthoquinone-2-sulfonate. In the latter case, the sulfur is recovered by filtration.
Processes for the extraction of hydrogen sulfide from gas are described by Oliveau in U.S. Pat. No. 4,844,876, by Elgue in U.S. Pat. No. 4,889,700, and by Hakka in U.S. Pat. No. 5,017,350. The latter patent also teaches the extraction of sulfur dioxide, inter alia.
Stiles in U.S. Pat. No. 5,023,063 and pending divisional application Ser. No. 07/694,085 filed May 1, 1991, now U.S. Pat. No. 5,176,888, teaches a method for catalytically reducing nitrogen and sulfur oxides in flue gases with reducing gases, e.g. hydrogen or hydrogen-carbon monoxide, to produce respectively nitrogen and water, and sulfur and water. Stiles suggests the use of cyclones, filters or electrostatic precipitation to remove the elemental sulfur thus formed, from the product gas stream. The product stream contains entrained sulfur vapor. Condensation to permit treatment of solid sulfur-gas mixtures by the means suggested, risks the formation of a range of particle sizes difficult to accommodate by the suggested cyclones or electrostatic precipitation. Further there is the very real danger of the deposition of sticky, amorphous sulfur on condensers or dephlegmators or of highly viscous sulfur which could not be processed by any of the suggested means.